Alkanolamine sweetening units are used for the removal of H.sub.2 S and CO.sub.2 from natural gases, enhanced oil recovery gases, refinery hydrodesulfurizer recycle gases, FCCU and Coker gas plant tail gases, LPG streams, and Claus sulfur recovery tail gases. The alkanolamines commonly used are ethanolamine, diethanolamine, methyldiethanolamine, diisopropanolamine, and triethanolamine. These compounds are weak bases in water solution. When solutions of alkanolamines are contacted in packed, sieve plate, bubble cap or valve tray columns with streams containing H.sub.2 S and CO.sub.2, the H.sub.2 S and CO.sub.2 dissolve into the alkanolamine solution. The following chemical reactions then take place: EQU H.sub.2 S+Aamine=AamineH.sup.+ +HS.sup.- EQU H.sub.2 O+CO.sub.2 +Aamine=AamineH.sup.+ +HCO.sub.3.sup.-
General Eqn.: Acid Gases+Alkanolamine=Alkanolamine Salts of Acid Gases
The solution of water, unreacted alkanolamine, and alkanolamine salts are subjected to steam stripping to decompose the alkanolamine salts and remove H.sub.2 S and CO.sub.2 from the alkanolamine. The H.sub.2 S and CO.sub.2 removed from the alkanolamine can then be processed by Claus sulfur recovery, incineration, fertilizer manufacture, or other means.
H.sub.2 S and CO.sub.2 are not the only gases in the above referred to streams which form weak acids when dissolved in water. Other such acid gases, as they are commonly called, that may appear in gas streams treated with alkanolamine include SO.sub.2, COS, or HCN. These gases also undergo the same reactions as H.sub.2 S and CO.sub.2 to form alkanolamine salts. These salts, though, cannot be removed by steam stripping as H.sub.2 S and CO.sub.2 salts are. Thus, they remain and accumulate in the system.
Another problem is presented if oxygen gets into the alkanolamine system. Oxidation of acid gas conjugate base anions leads to the formation of other alkanolamine salts most commonly salts of thiosulfate (s.sub.2 O.sub.3.sup.31 2), sulfate (SO.sub.4.sup.-2), thiocyanate (SCN.sup.31 ). Other inorganic acid anions such as chloride (Cl.sup.-) may also be present. In addition to the inorganic acid anions, the alkanolamine solution may also be contaminated with organic anions such as anions of formic and acetic acid and the like. The alkanolamine salts of these inorganic and organic anions also cannot be removed by steam stripping.
Alkanolamine salts which cannot be removed by heat, called heat-stable salts, reduce the effectiveness of alkanolamine treating. The alkanolamine is protonated and cannot react with either H.sub.2 S and CO.sub.2 which dissolve into the solution. Also, accumulated alkanolamine salts are known to cause corrosion in carbon steel equipment which is normally used in amine systems. The salts are also known to cause foaming problems which further decrease treating capacity.
The normal procedure used to deprotonate the alkanolamine, so it can react with H.sub.2 S and CO.sub.2 is to add an alkali metal hydroxide such as NaOH to the amine solution. The deprotonated alkanolamine can then be returned to H.sub.2 S and CO.sub.2 removal service. However, the sodium salts of the anions of the heat-stable salts are also heat stable, are difficult to remove and thus accumulate in the alkanolamine solution, with attendant corrosion and foaming problems.
In one process, the alkanolamine solution containing heat-stable alkali metal salts is contacted with an anion exchange resin to remove the heat-stable anions from the solution and thereafter the solution is contacted with a cation exchange resin whereby alkali metal ions are removed from the solution. Anions of any heat-stable alkanolamine salts are also removed by the anion exchange resin. Removing the heat-stable salts in this manner reduces foaming losses, corrosion and maximizes the alkanolamine concentration.
The anion exchange resin used in the described process is regenerated by flushing with water to remove free alkanolamines, followed by elution with dilute sodium hydroxide to displace heat-stable salt anions with hydroxide ions and a second water wash to remove residual sodium hydroxide and sodium salts. The cation exchange resin is regenerated by flushing with water to remove free alkanolamine, followed by elution with dilute hydrochloric acid to displace sodium cations with hydrogen ions. A second water wash is then used to remove residual hydrogen chloride and sodium chlorides.
In the described process, alkanolamine in the alkanolamine solution is protonated by hydrogen at the ionic sites on the cation resin and becomes attached to these sites as alkanolamine cations. When the cation resin is regenerated with the dilute hydrochloric acid, both alkali metal cation and such alkanolamine are displaced from the resin with hydrogen ions taking their place. The alkanolamine in the regenerant stream cannot be returned to the alkanolamine circulating system for reuse because the alkali metal and chloride ions in the regenerant would recontaminate the system.
In one method of solving this problem, the cation exchange resin containing alkali metal cations and alkanolamine cations is regenerated by eluting the resin with a dilute alkali metal hydroxide solution to displace the alkanolamine from the resin with minimal displacement of alkali metal cations. Displaced alkanolamine cations react with hydroxide ions to free alkanolamine (plus water) which is reused in the alkanolamine treating process. Thereafter, the resin is eluted with a weak mineral acid to displace the metal cations and any remaining alkanolamine from the resin. Preferably the resin is washed with water before and after each of the elution steps.
In another process, the anion exchange resin previously mentioned is not used. The alkanolamine solution containing free alkanolamine, alkali metal salts of heat-stable acid anions and any remaining alkanolamine salts of such anions instead is brought in contact with a cation exchange resin. In the process, the hydrogen ions on the resin are displaced with alkali metal cations. In addition, free alkanolamine in the alkanolamine solution is protonated with hydrogen ions on the resin and becomes attached to the resin as alkanolamine cations. The anions that were counter ions for the alkali metal cations associate with the hydrogen released from the resin to form acids which are removed from the system in the liquid passing through the resin and in the water wash which follows the contact step. Thereafter, the procedure as previously described is followed, viz. elution of the cation resin with dilute alkali metal hydroxide, followed by weak mineral acid elution with appropriate water washes.
Alkanolamine losses and excessive consumption of regeneration chemicals result from the lack of reliable and convenient procedures for detecting critical break points in the regeneration of cation exchange resins used for the removal of cations from alkanolamines. It would be desirable to have a process for determining the critical break at various stages of the treating process to reduce costs associated with poor treating of amine streams. It would also be desirable to monitor and control alkanolamine reactivation processes in which cation resins are regenerated to separate cations from alkanolamines.